The present invention relates to a method and system for monitoring the integrity of electrical insulation, particularly on a continuous basis while the electrical device protected by such insulation is in use.
While the insulation provided around electrical power conductors and equipment is designed to prevent the appearance of short circuits or excessive currents between members which are at different potentials, and to protect personnel against shock hazards, and the insulation provided for a particular system can be expected to perform this function as long as it is in good condition, insulation can experience deterioration which, after a period of time, will destroy its protective abilities. Such deterioration is particularly likely in the case of power cable systems or equipment which are not stationary, in which case they are subjected to flexing, impact, abrasion, etc., which will accelerate the deterioration of the insulating system.
The problems posed by such deterioration are particularly acute in the case of cable systems and equipment utilized in or near water, and especially for those systems which are in contact with sea water, which has an extremely low electrical resistance. Thus, adequate protection of equipment and personnel in such an environment requires special measures.
One possibility which has been suggested involves the use of a "floating" power system, i.e. one having no ground, so that the first ground fault will have no hard wire return path. However, this approach is not completely reliable because particular combinations of conductor line length, power frequency, and operating voltage normally have high distributed capacitive and resistive leakage currents which can mask or hide hazardous current levels which might occur at isolated points of insulation degradation. The appearance of such currents cannot be reliably monitored by ground leakage current detectors because normal variations in distributed leakage currents exceed the current level which is hazardous if it occurs at a fault point. In particular, when the normal capacitive coupling which exists between power conductors and sea water is high, previously existing detectors cannot be usefully set to take into account the fact that changes in the length of cable immersed in the water, particularly when the cable is being payed out or hauled in, produce a corresponding change in the value of the distributed capacitive leakage current through the insulation between the cable conductors and the sea water ground.
It has also been suggested to eliminate shock hazards by enclosing a cable in a conductive armor which will act to carry ground fault currents. However, a conductive armor which is substantial enough to carry anticipated ground fault currents without creation of an unacceptably high voltage between the longitudinal ends of the armor would be so heavy and stiff as to prevent handling and winching of the cable on a practical basis. Moreover, even conductive armor is subject to deterioration and failure, which would destroy its ability to carry ground fault currents but would normally go undetected.
It is currently a common practice to provide double insulation in those systems where maintenance of the integrity of the insulation is particularly important. However, even double insulation is subjected to deterioration due to wear, age and damage, which can render the insulation unsafe, but which, here again, will not be detected by prior art techniques until damage or injury occurs.
In addition, various schemes have already been proposed for interrupting a ground fault path through the use of circuit breakers which operate upon the first appearance of a ground fault current. Most of these systems can be arranged to protect equipment, but do not fully protect personnel against shocks.
A recently proposed system of the latter type, which is designed for use in house power systems, compares the currents in the "hot" and return lines of such a system and operates to break the current circuit if an unbalance should occur, as for example if insulation should become defective and a person should become a part of the current flow path, as by touching a bare wire on the "hot" of the power line. This system senses the current change resulting from such contact, and interrupts the power system in response thereto. However, it does not provide any forewarning of deterioration of the conductor insulation, and does not operate until a person comes in contact with a bare spot of the conductor.
While such devices may prove beneficial in house wiring systems, they are unusable in a practical basis for long conductor systems, high voltage or high power systems, or in subsea applications, because underwater cables and equipment are normally not grounded, and always present a relatively high normal leakage current. Furthermore, devices of this type are capable of operating only with single phase power systems, and cannot be arranged to operate with multiphase systems.
Procedures are also known for measuring deterioration of cable insulation between uses. For this purpose, a particular cable must be removed from its working environment and tested in a controlled environment by the application of special voltages. Of course, this procedure does not permit incipient shock hazards to be detected between tests.